1. Field of the Invention
The present invention relates to a semiconductor memory device.
2. Description of the Related Art
Flash memories have conventionally been known as electrically alterable non-volatile memories. A flash memory includes a memory cell array formed of NAND-connected or NOR-connected memory cells each having a floating gate structure. Ferroelectric memories are also known as non-volatile memories allowing high random access.
Resistive switching memories using variable-resistive elements in memory cells are proposed as a technique to further miniaturize the memory cells. Some of known variable-resistive elements are: a phase-change memory element that changes the resistance by switching a chalcogenide compound between the crystalline state and the amorphous state; an MRAM element that uses the tunnel magnetoresistance effect to change the resistance; a memory element of polymer ferroelectric RAM (PFRAM) whose resistive element is made of a conductive polymer; and a ReRAM element that changes the resistance with supply of voltage pulse (see, for instance, Japanese Patent Application Publication No. 2006-344349).
However, since a memory cell using a ReRAM element has an inherent instability, simply supply of voltage pulse sometimes fails to cause a resistance change of the memory cell, or sometimes causes a resistance change thereof in a direction reverse to an intended direction. If such resistance change in the reverse direction is left uncontrolled, adverse effects may occur such as a shorter service life of the memory cell.